Less than 1% of the total body magnesium is present in the peripheral blood. We developed assays to determine the magnesium content and concentration of mononuclear blood cells as a potential index for intracellular magnesium and/or total body magnesium status. Patients with a surgical cure of hyperparathyroidism show a significant increase in the magnesium concentration of red blood cells and the magnesium content of mononuclear blood cells. Patients with hypoparathyroidism have a significant elevation of magnesium in red blood cells and mononuclear blood cells compared with normal individuals. Further, intravenous infusion of magnesium acutely effects a significant increase in the magnesium content of mononuclear blood cells.